The present invention concerns a method for the hydrophilic treatment of an aluminum surface. More specifically, it concerns a method for hydrophilic treatment of the surface of the heat transfer fins of aluminum heat exchangers.
In the past technology for the surface treatment of heat exchangers made of aluminum or aluminum alloy (hereinunder referred to as (aluminum)) for the purpose of preventing white rust development, employed anodic oxidation coating, Boehmite coating, resin treatment etc. However, the coating formed by such methods provide the surface with almost no water wettability. For the purpose of preventing white rust development, chromate conversion coating is also used in practice, which also gives water wettability but to a slight extend and only for a short period after the formation of coating. Therefore, chromating alone cannot provide sufficient hydrophilic quality. Further, a chromate conversion coating has a tendency to lose its hydrophilic property as time passes, particularly under heating/drying conditions. Therefore, chromate coating for the surface treatment of heat-exchanger fins is not satisfactory.
It is of course, desirable that the efficiency of heat radiation/cooling of the heat exchanger be as high as possible. For this purpose, the radiation and cooling parts are designed to have as large a surface area as posasible for the majority of cases and consequently the interfin distance is made extremely narrow. This causes atmospheric moisture to condense on the surface of the heat exchanger when used for cooling, particularly at the interfin spaces. Water thus condensed forms water droplets to a greater and greater extent the more hydrophobic the fin surface is. Such water droplets fill up the interfin spaces resulting in increased air flow resistance and the heat exchange efficiency is thereby reduced.
Moreover, due to the air flow, water droplets in the interfin spaces are subject to spattering the area around the heat exchanger.
Various methods have been proposed for the purpose of preventing water droplets from forming at interfin spaces. Treatment of the surface with a silicate such as water glass is effective for improving the water wettability and heat resistance with low cost, in view of which a number of methods have been proposed.
In summary, the methods hitherto used involving both inorganic and organic compounds for coating formation can be classified as follows:
a: Phosphate treated aluminum surface is processed directly with aqueous silicate coating and then dried. This method is exemplified by the Japanese Laid-Open Patent Sho No. 50-38645 (1975). PA1 b: This method is exemplified by the Japanese Laid-Open Patent Sho No. 60-221582 (1985) which discloses that a hydrophilic inorganic coating comprising silicate, Beohmite etc. is formed on the aluminum sheet, over which a hydrophilic organic polymer having a ratio of polymerization more than 50 is coated. PA1 c: The aluminum surface treated with an organic polymer coating is followed by silicate liquid coating and drying, one example of which is described in Japanese Laid-Open Patent Sho No. 59-205596 (1984) which discloses a fin material where aluminum sheet is coated with an organic resin film having corrosion resistance over which a hydrophilic coating consisting of silicates such as silica sol, silicic acid and water glass is formed. PA1 d: The aluminum surface is coated with a mixture of organic polymer and inorganic silicate. This is exemplified by the following Laid-Open patents. PA1 Japanese Laid-Open Patent Sho No. 61-8593 (1986) discloses a fin material which is coated with a mixture consisting of styrene/maleic acid copolymer, polyacrylamide, butylene/maleic acid copolymer, polyacrylic acid or their salts and silicates as represented by xM.sub.2.ySiO.sub.2 (M=Li, Na or K, y/x.gtoreq.2). Japanese Laid-Open Patent Sho No. 60-101156 (1985) discloses a chemical for forming hydrophilic coating on aluminum which contains alkali silicate and carbonyl compounds (aldehyde, esters, amides etc.). PA1 With regard to other conventional methods that use organic compounds for the hydrophilic treatment of aluminum, Japanese Laid-Open Patent Sho No. 59-205596 discloses a method of using an organic solvent. Organic compounds disclosed therein are acrylic resins, epoxy base resins, urethane base resins, vinyl type resins such as polyvinyl chloride-vinyl acetate, polyethylene, polypropylene and the like, stylol base resins, phenolic resins, fluoro-resins, silicone resins, diaryl phthalate resins, polycarbonate resins, polyamide resins, alkyd resins, polyester resins, urea melamine resins, polyacetal resins, cellulose resins etc. These compounds may be combined with an organic solvent. On the other hand, the abovementioned Laid-Open Sho No. 60-101156 describes low molecular organic compounds having the carbonyl group (e.g. glyoxal) together with a water-soluble organic polymer (e.g. copolymer of acrylamide and acrylic acid) diluted with water and used for coating aluminum followed by heating and drying. PA1 The past technologies which use polyacrylamide as an organic compound for hydrophilic treatment have been further reviewed. PA1 As described in Japanese Laid-Open Patents Sho No. 61-101156 and Sho No. 61-8598, the use of a polyacrylamide as a chemical for hydrophilic treatment is known. Such compound can be uniformly mixed in water when the content is low. However, with increasing concentration caused during the drying process, the alkali silicate and polymer become separated into two phases, often resulting in non-reproducibility of the quality. This is a major drawback. PA1 Japanese Laid-Open Patent Sho NO. 60-221582 proposes a method that, over the film of a hydrophilic inorganic coating, polyacrylamide as hydrophilic organic coating former is applied wherein the degree of polymerization is adjusted so that solvent degreasing of press oil used for press working can be done with ease and the organic polymer layer remaining after degreasing can serve to fill pinholes of the inorganic coating. According to this patent, further, a cross-linking agent consisting of compounds of Zr, Ti etc. can provide said polyacrylamide with cross-linking of hydrophilic group in a range that such group is not entirely all cross-linked. According to said patent, the hydrophilic coating remaining as a final coating after solvent cleaning is a layer of inorganic, hydrophilic coating obtained by silicate treatment or Boehmite treatment as the undermost layer. PA1 (A): amphoteric polymer obtained by the copolymerization of monomer (I) which is represented by the empirical formula, ##STR1## where R.sub.1 represents H or CH.sub.3 and R.sub.2 and R.sub.3 represent H, alkyl radical of C.sub.1 -C.sub.4, benzyl radical or alkanol group of C.sub.2 -C.sub.3 -- together with an unsaturated cationic monomer (II) and an unsaturated anionic monomer (III). PA1 (B): amphoteric polymer obtained by the post treatment of homopolymer or copolymer of the abovementioned monomer (I). It may be desirable to include a water-soluble cross-linking agent (C) in the aqueous solution. In some cases, it may also be desirable to apply an aqueous water-glass solution over the above coatings and dry it to form a hydrophilic film. PA1 2-hydroxy methacrylate PA1 Diacetone acrylamide PA1 Methylol acrylamide PA1 Acrylol morpholine PA1 Acrylonitrile PA1 Methacrylic ester PA1 Styrene PA1 Vinyl acetate PA1 x.sup..crclbar. =acid radical of inorganic or organic acid. PA1 (i) Conversion of amide radical to carboxyl radical by hydrolysis. PA1 (ii) Conversion of amide radical to ##STR6## by Mannich reaction with ##STR7## and formaldehyde wherein R.sub.4, R.sub.5 are alkyl radical of C.sub.1 -C.sub.4, alkanol group of C.sub.2 -C.sub.3 or benzyl. PA1 (iii) Introduction of --CONH--R.sub.6 --NH.sub.2 by the Hofmann reaction between side-chain ester radical and alkylenediamine (H.sub.2 N--R.sub.6 --NH.sub.2), wherein R.sub.6 is alkylene radical of C.sub.2 -C.sub.6. PA1 (iv) Conversion to quaternary amino compound, by alkylating the amino radical of the reaction product of (ii) or (iii). PA1 Contact angle PA1 A water droplet of 1-2 mm diameter was placed on a coated surface and its contact angle was measured by face contact angle measuring apparatus, Model CA-P, product of Kyowa Kaimenkagaku Co., Ltd. Both a freshly coated surface and one subjected to marine water immersion for 1 week were tested. PA1 Corrosion resistance PA1 In accordance with JIS Z-2371 for salt spray test, the time for white rust development on 5% surface area was indicated. PA1 Running water resistance PA1 The test piece was immersed in running water at room temperature for 8 hr. and then dried at 80.degree. C. for 16 hr. After repeating this cycle 5 times, the contact angle of water was measured.
The first category (a) of conventional methods for hydrophilic film formation, provides the coating with a hydrophilic property but not with corrosion resistance. For this reason, such coating formation may lead on the contrary to degraded corrosion resistance and exhibits a disadvantage that white rust tends to occur.
In the case of the second category (b) of conventional methods for hydrophilic treatment, the hydrophilic property is mainly given by the organic components such as Boehmite and silicate. The main purpose of the organic coating is to prevent the abovementioned inorganic coating for being contaminated with press oil and thereby being made water repellent; after having performed this role, such organic coating is removed together with press oil in the subsequent degreasing stage. As a result, the performance is not satisfactory in the corrosion resistance nor in the hydrophilicity.
The third category (c) is satisfactory for corrosion resistance and for the hydrophilic performance during the incipient period, its disadvantage being, however, that the silicate of the top layer tends to be washed away as water condenses on the surface during operation resulting in degradation of the hydrophilic property.
In the fourth category (d), since silicate is contained in the coating it tends to be washed away and the fin treated with such method has a tendency to increasingly undergo white rusting. Further, the drying stage may cause the silicate and the organic polymer to separate into different layers and as a result the performance becomes variable largely depending on manufacture conditions, and in many cases the fin thereby treated exhibits insufficient hydrophilicity.
The method of Japanese Laid-Open Patent Sho No. 59-205596 uses an organic polymer coating having resistance to water and corrosion as the base coating. Since this method uses organic solvent in many cases, problems arise from fire hazard and environmental safety as well as from the low hydrophilicity of the thereby formed base film, which makes it difficult to use aqueous water-glass solution for forming a thin and uniform top coat over the base coat.
It is notable here than the method of Japanese Laid-Open Patent Sho No. 60-221582 where polyacrylamide etc. are also described should be regarded as equivalent to the conventional technology classified into the first category in regard to the way of constituting the coating layer for the reason that polyacrylamide does not remain on the fin to any significant extent, meaning that no suggestion is made of using water-soluble organic polymer like polyacrylamide as a permanent coating of the fin.
The inventors hypothesized that a single layer of organic polymer that constitutes the coating, provided that the coating thus formed be given sufficient corrosion resistance and hydrophilicity (including durability resistance to running water) might be capable of overcoming the disadvantages mentioned in the first category.
Further, such a layer of organic polymer as abovementioned may then be coated with a silicate film by making use of the technology as mentioned in the 2nd category. It is also possible to form an organic coating of high hydrophilicity over a double-layered structure consisting of a base coat with high corrosion resistance and a uniform top coat with sufficient hydrophilicity. In this way, exposure of a hydrophilic layer former such as silica gel and water glass can be avoided thereby minimizing tool abrasion during subsequent working.